Optimized method for assembling two substantially planar parts

ABSTRACT

Optimized method for assembling two substantially flat parts. 
     To assemble two parts ( 10, 12 ) that are substantially flat using fixations ( 14 ) such as rivets or bolts, first the ratio R is calculated between tensile and bending stresses relating to the forces to be transmitted. The end line of the fixations ( 14 ) is then oriented along an angle α whose absolute value is such that 10.8 ln(R)+16&lt;|α|&lt;13.9 ln(R)+28 when the ratio R is 1 or more, and such that 16°&lt;|α|&lt;28° when the ratio R is less than 1. The absolute value of angle α does not exceed 90°.

TECHNICAL FIELD

The invention concerns a method for assembling two flat or substantiallyflat parts, such as plates or sections, by means of fixations such asbolts, rivets etc.

More precisely, the invention relates to the optimised assembly of twoparts designed to bear and transmit predetermined forces, either uniformor varying in time, which may differ from one assembly to another.

The method according to the invention applies to all assemblies ofsubstantially flat parts, whether metallic or in composite material,which use fixations such as rivets or bolts. It finds particularlyadvantageous use in aeronautics, in which this type of assembly iswidely used.

PRIOR ART

In an aircraft, bolted or riveted assembly is the most frequently usedassembly mode. A passenger transport or cargo aircraft comprises morethan one million rivets and close to 300000 bolts.

In assemblies of this type, fixations perform the functions of forcetransfer, sealing and the transmission of static electricity current andlightening.

The design of assemblies using bolts and rivets is therefore vital forthe performance of the entire structure of the aircraft. Any poordesign, would lead to a limited lifetime and weight excess.

In the current state of the art, the positioning of rivets and bolts ismade according to the usual practice of each aircraft builder withouthaving true recourse to any particular methodology.

DESCRIPTION OF THE INVENTION

The subject of the invention is precisely a method for assembling twosubstantially flat parts, metallic or in composite material, aimed atoptimising the positioning of the fixations used to produce thisassembly, so as to guarantee controlled, optimum lifetime.

In accordance with the invention, this result is obtained by means of amethod for assembling two substantially flat parts, using at least oneend line of fixations intended to transmit determined forces betweensaid parts, and oriented in the plane of the parts, characterized inthat it consists of calculating a ratio R between the tensile stressesσ_(T) and bending stresses σ_(F) relating to said forces, and oforienting said end line of fixations along an angle α relative to thedirection of the neutral fibre of said parts, the absolute value of saidangle α being such that: 10.8 ln (R)+16 <|α|<13.9 ln (R)+28 when theratio R is at least 1, and such that 16<|α|<28 when the ratio R is lessthan 1, the absolute value of angle α being no more than 90°.

The applicant has established by test-supported numeric simulations,that by positioning the first line of fixations along a direction αpaying heed to the above-defined range of values, it is possible tobetter distribute the stresses borne by each of said fixations, andconsequently to increase the lifetime of the assemblies in respect offatigue and static resistance.

Under one preferred embodiment of the invention, the end line offixations is oriented along an angle α substantially equal in absolutevalue to 11.6 ln (R)+21 when the ratio R is at least 1, andsubstantially equal to 20° when the ratio R is less than 1.

If the forces to be transmitted are alternate forces, the end line offixations is advantageously oriented along the above-mentioned angle αand along an angle −α, either side of the neutral fibre of the parts.The fixation furthest in front is then on the neutral fibre.

The assembly method of the invention may be used both when parts areassembled directly one to the other and when they are assembled via oneor two fishplates.

In the former case, the two parts are directly assembled one to anotherby at least two end lines of fixations oriented along angle α.

In the latter case, that is to say when the two parts are assembled oneto another via a fishplate, each of the parts is fixed to the fishplateby at least two end lines of fixations. Advantageously, these end linesare then oriented along angle α. As a variant, at least the end linesthe furthest away from the other part are oriented along angle α.

In the third case, that is to say when the two parts are assembled oneto the other via two fishplates, each of the parts is fixed to the twofishplates by at least two end lines of fixations, among which at leastthe end line the most distant from the other part is oriented alongangle α.

The invention applies both to parts having a substantially constantthickness in the assembly zone and to parts whose thickness taperstowards the ends in the above-cited zone.

SHORT DESCRIPTION OF THE DRAWINGS

As limitative examples, different embodiments of the invention aredescribed below with reference to the appended drawings in which:

FIG. 1 is an overhead view giving a diagram of an assembly by rivets orbolts using the method of the invention;

FIG. 2 is a curve showing changes in angle α, formed by the first lineof rivets or bolts with the neutral fibre of the parts, relative to theratio R between the tensile stress σ_(T) and the bending stress σ_(F)according to the invention;

FIGS. 3A to 3D are perspective views which illustrate differentapplications of the invention to assemblies with no fishplate, with onefishplate and with two fishplates respectively; and

FIG. 4 is an overhead view comparable to FIG. 1, illustrating theparticular case in which the forces applied to the assembly arealternate forces.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

As schematically shown in FIG. 1, the invention concerns the assembly oftwo parts 10, 12 by means of a plurality of fixations 14, generallyformed of rivets or bolts. The two parts 10 and 12 so assembled may bemade up of any substantially flat parts. By “substantially flat parts”is meant here, as in the remainder of the disclosure, any parts such asplates or sections having a flat or close to flat geometry.

The method of the invention concerns the assembly of parts 10 and 12which, along their plane, are intended to undergo forces orpredetermined loads during subsequent use. These forces may be ofdifferent types depending upon the application concerned. In particular,the forces applied to the parts may be wave forces always in the samedirection or alternate (that is to say alternately in one direction,then in the other). The particular case of alternate forces will betreated below with reference to FIG. 4.

Irrespective of the forces applied to the parts, for each case it ispossible to determine a tensile stress σ_(T) corresponding to thetensile force N (FIG. 1) able to be applied between the parts alongtheir neutral fibre, and a bending stress σ_(F) corresponding to thebending moment M_(F) (FIG. 1) able to be applied between the parts.

As shown by the stress profile illustrated in the top part of FIG. 1, ifthe stresses borne by each of the assembly sides (left (G) and right (D)respectively in the figure) are designated σ_(G) and σ_(D), the tensilestress σ_(T) equals $\frac{ó_{G} + ó_{D}}{2}$

and the bending stress σ_(F) equals $\frac{ó_{G} - ó_{D}}{2}$

According to the invention, the ratio R between the tensile stress σ_(T)and the bending stress σ_(F) is determined by calculation. In mostapplications, the ratio R varies between a minimum value Rmin and amaximum value Rmax. In some cases, the ratio R may however have asubstantially constant value.

On the basis of the R ratio determined in this manner, the value of anangle α (FIG. 1) is fixed between a first line of fixations 14 and thedirection of the neutral fibre of parts 10 and 12 to be assembled. Forthis purpose, the curves in FIG. 2 are used which respectively show themaximum α_(max), minimum α_(min) and optimal values α_(opt) of angle αin relation to ratio R.

When the ratio R is 1 or more, the maximum value of angle α, whichcorresponds to the curve α_(max) in FIG. 2, is given by the relationshipα_(max)=13.9 ln(R)+28. Under the same conditions, the minimum value ofangle α, which corresponds to the curve α_(min), is given by therelationship α_(min)=10.8 ln(R)+16. Finally, still in cases when theratio R is 1 or more, the optimal value of angle α, which corresponds tothe curve α_(opt) in FIG. 2, is given by the relationship α_(opt)=11.6ln(R)+21.

When the ratio R is less than 1, the optimal value of angle α isapproximately 20°, the maximum and minimum values then being 28° and 16°respectively. All these values also correspond to those given by thecurves in FIG. 2.

In the more general case when R varies between two values Rmin and Rmax(these values are respectively 5 and 15 in the numeric exampleillustrated in FIG. 2), the value chosen for angle α must be such thatα_(min), (Rmax)≦α≦α_(max) (Rmin). In practice, a value for α is chosenlying substantially midway between these two terminals, that is to saythat α is given a value that is substantially equal to$\frac{{\alpha_{\min}( {R\quad \max} )} + {\alpha_{\max}( {R\quad \min} )}}{2}.$

Advantageously, if the ratio R is more or less constant, angle α isgiven a value substantially corresponding to the value given by thecurve α_(opt) for this value of R.

It is to be noted that the value given to angle α never exceeds 90°.Therefore, in the eextreme case in which the forces to be transmitted bythe assembly relate to simple tensile forces, angle α is preferablygiven a value of 90°, the minimum value, in this case, being 80°.

When angle α is given the optimal value α_(opt) the end lines offixations 14 oriented along this angle are arranged in optimal manner,such that the fixations of these lines are isocritical. The stressesborne by fixations 14 carrying the greatest load are then minimal. Thischaracteristic can therefore impart an optimal value to fatiguelifetime. This characteristic also optimises static resistance. Theseproperties subsist for as long as the value of angle α remains withinthe range delimited by angles α_(max) and α_(min).

The arrangement of the end lines of fixations 14 according to theinvention applies irrespective of assembly type.

Therefore, FIG. 3A shows the case of a simple assembly, in which the twoparts 10 and 12 to be assembled are directly fixed one to the otherwithout any fishplate.

The substantially flat ends of parts 10 and 12 then overlap so as to beassembled one to the other by two end lines LE1 and LE2 of fixations 14.As illustrated by way of example in FIG. 3A, the assembly generally alsocomprises intermediate fixations 15. In this type of assembly, the endlines LE1 and LE2 are oriented along angle α in accordance with theinvention. Also, fixations 14 and 15 are generally aligned in a certainnumber of rows oriented parallel to the neutral fibre of parts 10 and12. The number and arrangement of intermediate fixations 15 aredetermined in accordance with rules of the art.

FIG. 3B shows a first application of the invention to the assembly oftwo parts 10 and 12 via a fishplate 16.

In this case, each of parts 10 and 12 is fixed to fishplate 16 by anassembly comparable to the one which joins parts 10 and 12 in theapplication shown in FIG. 3A. In other words, parts 10 and 12 are placedend to end and the fishplate 16 covers the end of each part, being fixedto these ends by two end lines LE1 and LE2 of fixations 14 and byintermediate fixations 15. As in the previous case, the number and thearrangement of intermediate fixations 15 are determined in conventionalmanner using the rules of the art.

FIG. 3C also concerns the assembly of two parts 10 and 12 by means of afishplate 16.

As in the preceding case, fishplate 16 is directly fixed to each ofparts 10 and 12 by two end lines LE1 and LE2 of fixations 14 and byintermediate fixations 15. In this case, however, only the end lines LE1the most distant from the other part are oriented along angle α inaccordance with the invention. On the contrary, the end lines LE2 thenearest to the other part are oriented along an angle close to 90°relative to the neutral fibre of the two parts.

In this case, the fishplate 16 must be thickened in the zone of linesLE2 which are critical sites for the onset of cracks.

FIG. 3D shows the case in which parts 10 and 12 are assembled by meansof two fishplates 16, positioned either side of the ends of the parts.The two fishplates 16 are then joined separately to the end of each ofparts 10 and 12, said parts being arranged end to end as in FIGS. 3B and3C. As in the case shown in FIG. 3C, the joining of the fishplate toeach of the parts is ensured by two end lines LE1 and LE2 and by atleast one intermediate line L1 of fixations 14, and only the end lineLE1 the most distant from the other part is oriented along angle aaccording to the invention.

In the case just described with reference to FIG. 3D, in which parts 10and 12 are assembled one to the other by two fishplates 16, thethickness of each fishplate is substantially 0.6 times the thickness ofthe single fishplate used for the case shown in FIG. 3B.

As illustrated by FIGS. 3A to 3D, the ends of parts 10 and 12 and offishplates 16 if used, are advantageously cut parallel to end lines LE1and LE2 of fixations 14. With this arrangement, it is possible to limitthe weight of parts to minimal values.

In FIGS. 3A to 3D, the ends of parts 10 and 12 used for their assemblyare of uniform thickness corresponding to the thickness of said partsoutside the assembly zone. However, other arrangements are possiblewhile remaining within the scope of the invention. Therefore the ends ofparts 10 and 12 may also be of varying thickness which decreases eitherregularly in tapered manner or in stages towards the end of thecorresponding part.

As shown schematically in FIG. 4 and as previously mentioned, theinvention also applies to cases in which the forces which must betransmitted by the assembly are alternate forces, that is to say forcesalternately oriented in one direction and then in the other. Thesealternate forces may in particular be bending forces and/or tensileforces. In this case, the end line formed by fixations 14 is orientedalong angle α on one side of the neutral fibre of parts 10 and 12, andalong an angle −α on the other side of said neutral fibre.

The invention is evidently not limited to the embodiments justdescribed. It may be applied to parts made in identical or differentmaterials, in metallic or composite materials. It may also be appliedboth to parts that are substantially flat and to sections assembled viatheir substantially flat cores.

What is claimed is:
 1. Method for assembling two substantially flatparts by means of at least one end line of fixations intended totransmit determined forces between said parts and oriented along theplane of the parts, in which said method consists of calculating atleast one ratio R between the tensile stresses s_(T) and bendingstresses s_(F) relating to said forces, and of orienting said end lineof fixations along an angle α relative to the direction of the neutralfibre of said parts, the absolute value of said angle α being such that:α_(min)<|α|<α_(max) where α_(min)=10.8 In(R)+16 and α_(max)=13.9In(R)+28 when the ratio R is 1 or more, and in which α_(min)=16 andα_(max)=28 when the ratio R is less than 1, the absolute value of angleα being no more than 90°.
 2. Method according to claim 1, in which whenthe ratio R varies between a minimal value Rmin and a maximum valueRmax, said end line of fixations is oriented along an angle α whoseabsolute value is such that α_(min) (Rmax)≦|α|α_(max) (Rmin).
 3. Methodaccording to claim 2, in which the absolute value of angle α is given avalue substantially equal to$\frac{{\alpha_{\min}( {R\quad \max} )} + {\alpha_{\max}( {R\quad \min} )}}{2}.$


4. Method according to claim 1 in which, when the ratio R is more orless constant, said end line of fixations is oriented along an angle asubstantially equal, in absolute value, to α_(opt) in which α_(opt)=11.6 ln(R)+21 when α_(opt) is 1 or more and with α_(opt) =20° when R isless than
 1. 5. Method according to claim 1, in which the forces to betransmitted being alternate forces, the end line of fixations isoriented along said angle α and along an angle α either side of theneutral fibre of parts.
 6. Method according to claim 1, in which the twoparts are assembled directly one to the other by at least two end linesof fixations oriented along said angle α.
 7. Method according to claim1, in which the two parts are assembled one to the other via afishplate, each of the parts being fixed to the fishplate by at leasttwo end lines of fixations oriented along said angle α.
 8. Methodaccording to claim 1, in which the two parts are assembled one to theother via a fishplate, each of the parts being fixed to the fishplate byat least two end lines of fixations, at least the end lines the mostdistant from the other part being oriented along said angle α.
 9. Methodaccording to claim 1, in which said parts are assembled one to the othervia two fishplates, each of the parts being fixed to the two fishplatesby at least two end lines of fixations, among which at least the endline the most distant from the other part is oriented along said angleα.
 10. Method according to claim 1, in which the thickness of said partsis substantially constant in the assembly zone.
 11. Method according toclaim 1, in which the thickness of said parts decreases towards theirends, in the assembly zone.